ILoveHostas.net. is one of the better places to get Heuchera. They sell under the name Casita Azul on Ebay.
For other plants I like Santa Rosa Gardens, they have some really good sales after the spring rush is over, almost always have coupon codes and very nice plants for the money. They have a pre-order sale going on now. Only problem is not much of a Heuchera selection.
